Many screening assays, both for the detection of potential therapeutics and for the isolation of clonal populations, require cells to be cultured in microtiter plates. The small well size and volume of the 1536-well plate make conventional methods of culturing cells inadequate under certain conditions. Under those conditions, evaporation of media is too rapid to support cell growth.
While the volumes of typical sample plates having larger well volumes than those found in 96-well plates is such that standard approaches may be employed without too rapid evaporation of media to support cell growth, once the volume of individual wells begins to get below 10 .mu.l, evaporation of the media from the wells begins to be a problem. At the 1 .mu.l well volume typical of the 1536-well plate described in U.S. Provisional Patent Application Ser. No. 60/037,636 filed Feb. 18, 1997 and incorporated by reference herein, too rapid evaporation of well media can be a substantial problem.
This problem cannot necessarily be solved by simply increasing the humidity of the cell culture chamber. For example, at a temperature of 37.degree. C., it may be impossible to increase the chamber humidity above approximately 95% humidity. This limitation results because the sides of the chamber are typically sufficiently cooler than the chamber air temperature that condensation occurs and water comes out of the chamber atmosphere as quickly as it is added. As a consequence, at this temperature and maximum global humidity, too rapid evaporation of well media may still occur.
It will be recognized to be highly advantageous to provide a simple, cost effective solution to this and other problems which result when multiple well plates having low volume wells such as 1536-well plates are utilized in standard cell culture chambers.